multi_blocking: fix pessimization spotted by clang.
multi_blocking: fix pessimization spotted by clang.

funkey/vigra,dstoe/vigra,dagophil/vigra,dagophil/vigra,dstoe/vigra,dstoe/vigra,dagophil/vigra,dstoe/vigra,funkey/vigra,funkey/vigra,dagophil/vigra,funkey/vigra,dstoe/vigra,dagophil/vigra,funkey/vigra,dstoe/vigra,dagophil/vigra,funkey/vigra,dagophil/vigra,funkey/vigra,dstoe/vigra